1. Field of the Invention
The present invention relates to a light-emitting device, more particularly a light-emitting device utilizing a polycrystalline light-emitting diode of a III-V compound semiconductor, and a printer utilizing said semiconductor device.
2. Related Background Art
The conventional polycrystalline semiconductor materials have been applied in the following fields.
Among the polycrystalline semiconductor materials of the group IV of the periodic table, polycrystalline silicon has been principally used, for example, in solar cells and thin film transistors. Also among the polycrystalline compound semiconductors of the groups II-IV, those based on cadmium have been utilized thin film transistors and photosensors, and are partly investigated for application to solar cells. Also those based on zinc have been utilized in fluorescent materials and piezoelectric devices. Also recently the polycrystalline materials of chalcopyrite family, such as CuInSe.sub.2, are being investigated for application in solar cells.
Among the polycrystalline III-V compound semiconductor materials, those based on Ga or In were investigated for application in solar cells, but have not reached the level of commercialization.
In the field of the polycrystalline III-V semiconductor materials, various references are available on the application in solar cells, but the reports on the light-emitting characteristics are limited. Salerno et al. reported on the electron beam luminescence in Conf. RECIEEE, vol. 15, p. 1174-1178, but no report was made on the light-emitting diode characteristic utilizing a PN junction.
The display device utilizing light-emitting diodes (LED) is generally constructed by forming LED's on a monocrystalline wafer, cutting said LED's singly or in the unit of plural devices out of said wafer, and adhering such LED's on a supporting substrate, either in the form of an independent lamp or a display device for characters and symbols. Also as an LED display device of a large area, there have been produced devices containing plural LED's in a hybrid structure, but such large area devices are limited in application because of the high cost.
In order to overcome the limitation in the display area of such LED display devices, the present inventors proposed, in the Japanese Patent Laid-open Application No. 64-723, a selective nucleation method for producing a monocrystalline III-V compound semiconductor over a large area.
This method consists of utilizing a substrate having a non-nucleation surface with a low nucleation density for the III-V compound crystal and a nucleation surface of an amorphous material, which is positioned adjacent to said non-nucleation surface, has a sufficiently small area for allowing crystal growth only from a single nucleus and has a nucleation density higher than that of the non-nucleation surface, and growing the monocrystalline III-V compound crystal from said single nucleus extending beyond the nucleation surface and over the non-nucleation surface.
Also the present inventors proposed, in the Japanese Patent Laid-open Application No. 63-239988, an LED device utilizing this technology. Said proposal disclosed the formation of an LED on a non-monocrystalline substrate, by formation of a PN junction area through a change in the crystal forming conditions in the course of formation of said single crystal.
The polycrystalline substances produced by conventionally known methods have not been investigated for application to light-emitting devices such as LED, as they are not suitable for the preparation of such devices because of excessively small or uneven crystal grain sizes.
On the other hand, the above-mentioned selective nucleation method can provide a large-area III-V single crystal on a non-monocrystalline substrate, but there may sometimes appear polycrystals on the nucleation surface or a non-occupied state without crystals on the nucleation surface. Such substrate lacks uniformity when LED devices are formed thereon, as the light-emitting intensity becomes lower in the area of such polycrystals and becomes zero in the non-occupied areas.
Also the obtained single crystal sometimes shows strong anisotropy of growth, resulting in oblong abnormal growth, so that the device-making process such as electrode formation may become difficult.
Also the selective nucleation method has been associated with a contradicting drawback that a crystal growth condition enabling a high rate of single crystal formation results in a reduced occupancy rate, while a condition enabling a high occupancy rate results in a reduced rate of single crystal formation.
Because of the above-mentioned drawbacks, priority has often been given to the improvement on the production yield over a large area, with a certain sacrifice of luminance, unless a particularly high luminance is required for the devices.
Also as the crystal concentrically grown from the conventional nucleation surface has a limited tolerance for the mask alignment error in the device forming process after the crystal growth, and there has been longed for an improvement in the projection yield.